Oil and gas exploration and extraction require well drilling to ever increasing depths, which increases the challenges associated with providing power to measurement devices close to the drilling location and in telemetry repeaters at intervals along the drill string. Wireline measurement tools that require electrical cabling to be provided from the surface to extreme well depths cannot be used during drilling, and conventional batteries do not provide adequate power, are relatively expensive for their usable life, and the cost of periodically replacing depleted batteries is prohibitive because of the high cost of delayed drilling while a MWD tool is removed.
However, downhole generators are known that operate on the principle of extracting energy from the flow of drilling ‘mud’ through the bore of the drill pipe. As is well known in the art, a drilling fluid is pumped downhole to cool the drill bit and to provide lubrication and remove cuttings, and the flow of this fluid provides a source of energy that can be exploited to power electrical components.
Conventional flow-based generator systems involve turbine blades within the fluid flow pipe, which transmit energy via a central shaft to an on-axis electrical generator. A bladed stator may impart angular velocity to the fluid flow before it reaches the turbine blades.
A problem with these known solutions is that their arrangements of the flowgear (e.g. an arrangement in which the bladed stator and turbine blades project from an outer surface of the generator) make retrieval of the flowgear or the generator extremely difficult or impossible during downhole operations. Their wires for electrical power connection between the generator and tool casing contribute to this difficulty.
Thus, it is impossible to change the flowgear on conventional generators, either to replace damaged parts or to change the flow rate, without removing the generator from the tool string for re-work (typically in a district workshop away from the drilling rig). To continue drilling without major delays requires additional spare generators which represent a large capital asset investment, but the drilling delay inherent in tripping the drill string to remove the generator is very costly even if the generator can be promptly replaced. The inventors of the present invention have recognized that it is desirable to improve the ease with which flowgear can be interchanged, or indeed to provide a system where such changes are minimised or eliminated. Another problem with known generators implemented within a drill collar insert is that they typically fail to provide an adequate power output for some applications, and fail to provide a sufficiently stable output. It is also desirable to mitigate the other problems described above.